Abstract : Marine Synechococcus are an important component of the picoplankton in the Eastern Mediterranean. These cells are small and rod-shaped, usually about 0.8 X 1.0 micrometers in size. Because of their distinctive fluorescence characteristics, they can be distinguished from other types of phytoplankton and enumerated by epifluorescence microscopy. These cells are abundant in both the mixed layer and the thermocline of the Eastern Mediterranean, reaching maximum summertime densities of 10(exp 4) per ml (Li et al, 1993). These cells are so small that they are effectively neutrally bouyant in seawater and theoretical considerations suggest that the cells in the mixed layer move between the surface and the top of the thermocline relatively frequently whereas those in the thermocline remain within a relatively narrow depth interval for periods of days or even months (Lande and Wood 1987). Thus, cells in the mixed layer encounter a range of conditions of irriadiance, light quality, and nutrient availability over short time scales, and cells in the thermocline experience regular diurnal fluctionations in irradiance, but relatively little variation in light quality or nutrient avallability over physiologically long time scales. Since this implies that the thermocline and mixed layer are very different kinds of environments for phytoplankton, we have isolated clones of marine Synechococccus from the mixed layer and thermocline of a station in the northwestern Levantine Basin in order to determine if there are distinctive ecotypes of marine Synechococcus present in these two different regions of the water column. p1